Atomic-layer-deposited La₂O₃ films were grown on Si with different O₃ pulse times and growth temperatures. The interfacial reactions and impurity behaviors were observed using in situ X-ray photoelectron spectroscopy. Longer pulse time of O₃ formed the solid SiO₂ interfacial barrier layer, which suppressed La-silicate formation. Meanwhile, the carboxyl compound acting as an impurity phase was replaced with LaCO₃ on increasing the O₃ pulse time due to further oxidation and reaction of La. Higher growth temperatures enhanced La-silicate formation by mixed diffusion of Si and La₂O₃, during which most of the La₂O₃ phase was consumed at 400 °C. C and N impurities decreased with increasing growth temperature and completely disappear at 400 °C.